There is an increasing need for light-weight, high-output power supplies for powering the increasing supply of portable electronic equipment such as personal computers. Electrochemical cells are commonly used as power supplies for a variety of applications but are often impractical for use with portable electronic equipment because the electrical energy densities of the electrochemical cells are too low. In other words, conventional electrochemical cells that produce the desired power output are often too heavy for use with portable equipment.
One electrochemical cell that is an exception is the metal-air cell. Metal-air cells have a relatively high energy density because the cathode of a metal-air cell utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. This results in a relatively light-weight power supply.
Metal-air cells include an air-permeable cathode and a metallic anode surrounded by an aqueous electrolyte. Metal-air cells function through the reduction of oxygen which reacts with a metal to form an electric current. Typically, the oxygen is taken from the ambient air. For example, in a zinc-air cell, the anode contains zinc, and during operation, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. Both primary and secondary metal-air cells have been developed. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and cathode of the cell and reversing the electrochemical reaction. Oxygen is discharged to the atmosphere through the air-permeable cathode.
One problem with metal-air cells is that the difference between the ambient relative humidity and the internal relative humidity of the cell can cause the metal-air cell to fail. Equilibrium vapor pressure of the metal-air cell results in an equilibrium relative humidity that is typically about 45 percent. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air cell, the metal-air cell will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the cell to leak. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air cell, the cell will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery cell is used, failure occurs from drying out.
During operation of a metal-air cell, the electrolytic reaction produces heat and increases the temperature of the cell. The heat produced by the electrolytic reaction increases the rate of vaporization of the water contained in the cell and can increase the rate at which the cell dries out.
Drying out and flooding are even greater problems for secondary metal-air cells than for primary metal-air cells. Although ambient humidity may not be a sufficient problem to flood or dry out a cell after a single cycle, cumulative water gain or loss from a series of discharge and charge cycles can cause premature failure of a secondary metal-air cell. Another problem with metal-air cells is that contaminants in the air such as carbon dioxide, smoke, and sulfides, decrease the battery output. For example, carbon dioxide reacts with the metal-hydroxide in the electrolyte. The reaction between carbon dioxide and the metal hydroxide forms a metal carbonate compound that interferes with the electrochemical reaction.
Thus, it has been desirable to control the exposure of the air cathode in a metal-air cell to air so that the amount of air to which the cathode is exposed is sufficient to generate the power demands on the cell but insufficient to cause premature failure of the cell through flooding, drying out, or the accumulation of contaminants. It has been desirable to limit the amount of air over the cathode to an amount approaching the stoichiometric requirements required by the cell to produce the desired power output.
One method for controlling the exposure of an air cathode to air is disclosed in U.S. Pat. No. 4,118,544 which discloses a primary metal-air cell for powering a device such as a hearing aid. The metal-air cell disclosed in U.S. Pat. No. 4,118,544 discloses a metal-air cell having a thin layer of microporous material sandwiched tightly between a perforated cover and an air cathode. The perforations or apertures in the cover control the exposure of the air cathode to air to limit access of the air cathode to excessive moisture and carbon dioxide. Although such an arrangement is effective to limit the exposure of the air cathode to moisture and carbon dioxide, the exposure of the air cathode to air would be too restrictive if such an arrangement were applied to a larger, more powerful metal-air cell and result in inefficient use of the air cathode. In addition, in such an arrangement or an arrangement where the cathode cover is directly against the cathode, the oxygen from the ambient air tends to diffuse through the air cathode at the points of the apertures and create localized reactions. As a result, the oxygen does not react at areas between the apertures of the air cathode cover and the output of the cell is limited.
Another approach to controlling the exposure of metal-air cell cathodes to air is through the use of an air manager system. In an air manager system, a fan supplies air through a system of sized openings and plenums in a housing containing an array of metal-air cells. The exposure of the air cathodes of the cells to air is controlled by the rate of delivery of air by the fan and the sizes of the plenums and openings in the housing. Such arrangements have been effective; however, they have been costly to produce because of the necessity of air-tight seals and appropriate part tolerances to deliver the appropriate amount of air to the metal-air cells.
Therefore, there is a need for an effective, economical means for controlling the exposure of air cathodes in metal-air cells to air to provide a longer useful life for metal-air cells.